1. Field of the Invention
The invention relates to an electric vehicle motor control apparatus.
2. Description of the Related Art
In a conventional electric vehicle, a motor is driven by current supplied from a battery installed in the vehicle. Since the field current is supplied to the motor through the brush and the commutator that slide on each other, the conventional electric vehicle requires checks and maintenance of the brush and the commutator which are inconvenient and, in addition, the sliding therebetween produces unpleasant noises.
To eliminate these problems, an electric vehicle has been provided which uses a brushless DC motor comprising a permanent magnet rotor and stator coils. For the brushless DC motor, sine-wave signals of three phases, that is, U-phase, V-phase and W-phase, are generated corresponding to the magnetic pole position of the rotor. The sine-wave signals are sent together with a current instruct value to a current waveform control circuit. The current waveform control circuit generates a three-phase pulse-width modulated signal (PWM) having a pulse width corresponding to the current instruct value, which is sent to a base drive circuit. The base drive circuit generates a transistor drive signal corresponding to the pulse-width modulated signal, and outputs the signal to a bridge circuit (inverter). The bridge circuit, comprising six transistors, turns on the transistors to generate a phase current only while the pulse-width modulated signal remains on, and supplies the phase current to the stator coils. The phase current is then detected by a current sensor and fed back to the current waveform control circuit.
If the conventional electric vehicle stops on a steep uphill while the motor is being supplied with phase current, the motor is locked stopping the rotor. Since the sine-wave signal is held in a state corresponding to the rotor magnetic pole position in such a case, the pulse-width modulated signal is also held in a comparable state. Consequently, the on-off duty ratio of a specific one or more of the transistors becomes approximately 100% and the phase current continues flowing through those transistors, causing an excessively large load concentration on the transistors. If the motor continues operating at a low speed, as opposed to coming to stop, all the transistors continue to be on for a long time, during which the phase current continues flowing, thus causing a concentration of excessively large loads on the transistors.